Landfill shortages have made it increasingly more desirable to recycle materials heretofore disposed in landfills. For example, rubber tires and many plastic items are now being recycled for many new uses. One such use involves milling the recycled rubber or plastic, and mixing the resulting ground product with materials for paving roads. Indeed, U.S. federal legislation presently mandates the use of ground tire rubber in federally financed roads.
In conventional milling operations, including cryogenic milling operations, a rotor is used to provide the velocity or impact energy necessary to fracture the materials along microcracks or dislocations therein. For instance, rubber tire particles may be milled in a typical cryogenic hammermill to a ground rubber product, all of which is smaller than 30 U.S. mesh and 40% of which is smaller than 80 U.S. mesh. In order to achieve still smaller particle sizes, it is ordinarily necessary to increase the speed of the rotor within the mill so as to increase the impact energy imparted to the rubber particles. However, prior attempts to further reduce the particle size of material smaller than 30 U.S. mesh have resulted in lower throughput (i.e., pounds of material per hour through the mill) and in significantly higher consumption of the coolant, which is used to lower the temperature of the tire rubber particles to cryogenic levels.
In addition, the particle dislocations, which provide fracture sites and lead to brittle fracturing at relatively low impact velocities [less than about 400 feet per second ("FPS")], are often depleted at such a mesh size. Thus, further particle size reduction through the mechanism of brittle fracture requires greater rotor speeds to generate higher impact velocities. However, the speed at which the rotor moves is constrained by limitations of the material from which the rotor is constructed and the geometry or design of the rotor itself. For instance, at speeds approaching 600 to 800 FPS, conventional rotors will tend to shatter, and the bearings and seals used in connection with the rotor will tend to be destroyed.
A significant increase in rotor speed also causes increased windage or drag, leading to inefficiencies in the operation. More specifically, the increased windage reduces the power available to conduct the milling operation. For example, about 85 to 90% of the operating power of a mill whose rotor tip speed is about 600 FPS is consumed by windage.
Drag and friction created during operation of such mills also cause an increase in the temperature of the rotor and the temperature of the materials being milled. These temperature increases adversely affect the efficiency of the milling operation and the integrity of the milled materials.
It is known generally that, in high speed electrical machines having large diameter rotors, the inefficiencies caused by friction in air may be reduced by about 85 to 93% by conducting the operation in a hydrogen environment.
U.S. Pat. No. 4,645,131 discloses a powder mill which uses a vacuum to reduce the drag on metal powders. The powder mill of the '131 patent is said to mill the metal powders, which are cooled to a low temperature such as -100.degree. F., to a particle size of smaller than 20 .mu.m (625 U.S. mesh). The vacuum is reportedly used to reduce drag on the milled metal powders in an attempt to minimize interference with the milling operation, as particles of that size have little mass and the drag created under ambient conditions would tend to suspend the particles. In addition, the vacuum is used to combat the problem of metal oxidation, which occurs under atmospheric conditions and which becomes more pronounced as particle size decreases (and particle surface area increases).
One known mill, which was developed for heat-sensitive materials, is the Victory Mill, commercially available from Hosokawa Micron, Summit, N.J. This Hosokawa mill operates by impact pulverization, and is designed for coarse-to-medium size reduction of heat-sensitive materials (e.g., thermoplastics). This mill is also intended to be used without refrigerants and in an ordinary ambient atmosphere, though air cooling may be used. The rotor of the mill is designed to reduce friction between particles being milled, though it is not designed to operate at speeds approaching sonic.
Previously, where a particle size smaller than a certain mesh was desired, material exiting a mill could be passed through a screen or sieve of about that desired mesh to obtain material with a particle size smaller than that of the screen or sieve mesh. The remaining material (i.e., that which does not pass through the screen or sieve) could then be recycled into the feed material of that mill, or fed into a separate mill, for further size reduction. Such recycling creates inefficiencies in the milling operation insofar as the finer (i.e., smaller particle size) material tends to interfere with the milling of the larger particle size material. In addition, such use of a separate mill to further reduce particle size by a primary mechanism different than brittle fracture will decrease the throughput of the milled material. This results in increased expenditure (e.g., increased power consumption) to maintain the same degree of particle size reduction.
Another way in which particle size reduction is commonly carried out is wet milling, wherein horizontal serrated stone wheels and water dissipate the heat created during the grinding operation. However, with such a wet milling technique, water needs to be removed from the milled product. This plainly detracts from the efficiency of that technique.
Accordingly, there exists a need for an impact system which mills materials to fine particle sizes [such as to smaller than about 80 U.S. mesh (e.g., smaller than about 177 .mu.m)], without encountering the inefficiencies referred to above. It would be desirable for such a system to mill materials under a modified gaseous atmosphere so as to reduce windage and friction resulting therefrom. It would also be desirable for such a system to mill materials with a rotor operating at high tip speeds, and to enhance the brittleness of the materials to be milled under reduced temperature conditions, particularly under cryogenic temperature conditions.